The proliferation of mobile devices with large numbers of increasingly sophisticated and powerful functions has led to a rapidly increasing demand for high-power, high-capacity electrical energy storage devices. The situation will become even more acute when hybrid and battery-powered electric vehicles become a preferred mode of transportation. Currently, the combined need for high capacity and high power is met by two separate devices: a rechargeable battery for capacity and an ultracapacitor for power. A single device, such as a rechargeable battery that can operate at high power, is highly desirable because it would be lighter, simpler to control, and able to provide sustained high power.
Graphene, a two-dimensional aromatic monolayer graphite has recently been investigated with respect to rate capability and cycling stability when used in conventional electrochemical energy storage devices. The excellent tensile modulus and mechanical durability of self-supporting graphene materials eliminate the requirement for traditional inactive additives and metal foil current collectors. In addition, charge storage materials made by sandwiching high-capacity metallic or redox materials between graphene sheets have shown improved cycling stability.
Unfortunately, the practical charge storage capability of graphene based anodes in Li ion batteries at high charge/discharge rates has been constrained by the structure of graphene, which has a very high aspect ratio (i.e., it is wide, but very thin). In order to access the interior of a graphene stack, Li ions need to enter the structure at the edge of the stack and travel distances of the order of microns. Thus, at high power, when fast Li exchange between the electrolyte solution and the electrode is required, only the regions near the edge of the graphene stack are accessible.